Fleschner C R, Cenedella R J
Department of Biochemistry, Kirksville College of Osteopathic Medicine, MO 63501, USA.
Exp Eye Res. 1993 Jun;56(6):649-57. doi: 10.1006/exer.1993.1082.
A membrane fraction was isolated from the water soluble fraction of bovine lens by increasing the density of the water soluble fraction with KBr and subjecting it to overnight centrifugation at 100000 g. We have called this fraction, which floats to the top of the mixture upon centrifugation, the non-sedimenting membrane fraction (NSMF). Electron microscopy of the NSMF revealed that it is composed of the expected membrane structures of unit membrane, fiber junction and cytoskeleton. Significantly less of the total membrane of the NSMF was devoted to fiber junction (22.8%) than in the sedimenting membrane fraction (SMF) (41.1%) prepared by sucrose density centrifugation of the water insoluble fraction. The NSMF accounted for about 7-12% of the total bovine lens membrane, and preliminary experiments demonstrated that a similar fraction could be isolated from the water soluble fraction of lenses from rats, rabbits, chickens and humans. The NSMF contained about 0.9 mg total lipid per mg total membrane protein, which was significantly greater than the value obtained for the SMF (0.5 mg total lipid per mg total membrane protein). The greater relative amount of total lipid in the NSMF was due to a significantly greater relative amount of phospholipid in the NSMF which was further reflected by the observation that the cholesterol: phospholipid molar ration of the NSMF (0.58) was significantly less than that of the SMF (0.88). Thus the relative lipid composition of the NSMF was significantly different than that of the SMF. Although the phospholipid content of the NSMF was greater than that of the SMF, the compositions of the phospholipids in the two membrane fractions were similar. The NSMF possessed essentially the same polypeptides (both extrinsic and intrinsic) which were found in the SMF. The NSMF was found to be distributed throughout the lens in a proportionate manner. We conclude that the NSMF may account for most of the lipid which remains in the water soluble fraction of the normal bovine lens after sedimentation of the water insoluble fraction. This membrane fraction substantially differs from the SMF in terms of structure and relative lipid composition. We speculate that the NSMF may represent a specialised domain of the fiber cell plasma membrane which has been previously unrecognized.
通过用溴化钾增加水溶性部分的密度，并在100000g下进行过夜离心，从牛晶状体的水溶性部分中分离出一个膜部分。我们将这个在离心时漂浮到混合物顶部的部分称为非沉降膜部分（NSMF）。NSMF的电子显微镜检查显示，它由预期的单位膜、纤维连接和细胞骨架的膜结构组成。与通过水不溶性部分的蔗糖密度离心制备的沉降膜部分（SMF）（41.1%）相比，NSMF总膜中用于纤维连接的比例显著降低（22.8%）。NSMF约占牛晶状体总膜的7-12%，初步实验表明，从大鼠、兔子、鸡和人类的晶状体水溶性部分中可以分离出类似的部分。NSMF每毫克总膜蛋白含有约0.9毫克总脂质，这显著高于SMF（每毫克总膜蛋白0.5毫克总脂质）的值。NSMF中总脂质的相对含量较高是由于NSMF中磷脂的相对含量显著较高，这进一步体现在NSMF的胆固醇：磷脂摩尔比（0.58）显著低于SMF（0.88）的观察结果中。因此，NSMF的相对脂质组成与SMF显著不同。尽管NSMF的磷脂含量高于SMF，但两个膜部分中磷脂的组成相似。NSMF具有与SMF中发现的基本相同的多肽（包括外在和内在多肽）。发现NSMF以成比例的方式分布在整个晶状体中。我们得出结论，NSMF可能占正常牛晶状体水不溶性部分沉降后仍留在水溶性部分中的大部分脂质。这个膜部分在结构和相对脂质组成方面与SMF有很大不同。我们推测NSMF可能代表了以前未被认识的纤维细胞质膜的一个特殊区域。
